1. Field of the Invention
The present invention relates generally to light-emitting diodes (LEDs), and more particularly to light collection/distribution systems that utilize one or more LEDs.
2. Description of Related Art
Light emitting diodes (LEDs) are a widely available, inexpensive, and efficient light source. For low light uses such as camping headlamps, one or two LEDs provide adequate light. However, to utilize LEDs for applications that require more light, such as automobile headlamps, it is necessary to combine the outputs of a plurality of LEDs. The LED prior art is less than satisfactory regarding the combination of the luminous outputs of a plurality of emitter-chips. Physical chip-adjacency can indeed produce a larger light source, but heat-removal limitations reduce the total luminance. Also, there is little continuity of illuminance between the adjacent emitters, leaving dark zones between the individual emitters. LEDs are available from a wide variety of suppliers, and in commercially available LEDs the emitters themselves have pronounced variations in luminance. For example, some suppliers (e.g., the OSRAM Corporation of San Jose, Calif. and the Cree Corporation of Santa Barbara, Calif.) manufacture high-power LEDs with wires and bonding pads that block light from the top of the emitting chip. In contrast, high-power LEDs from the Lumileds Corporation of San Jose, Calif. exemplify flip-chips, which have no wires or bonds that would otherwise block light emission in front. Even these, however, show great luminance variations across the emitter. The Luxeon I and Luxeon III LEDs by Lumileds, for example, can vary in luminance by a factor of ten from center to edge, with random patterns in between that differ from one chip to the next. Such undesirable patterning, whether on flip-chips or front-wired chips, can cause detrimental artifacts in the beams of collimating or condensing lenses. Although diffusers can be placed over such lenses, diffusers lose 15% of the light and give the beam a fuzzy edge. A more efficient method of source homogenizing, one that preserves sharp edges, would be a significant advance in illumination optics. Although thin-film LEDs have greatly improved uniformity over conventional on-substrate LEDs, there are fundamental reasons why they will always have nonuniform illuminance, because of inherently nonuniform current distribution downward through the active, light-generating layer. Using larger soldered electrodes causes more useless surface recombination at their juncture with the LED, so that electrodes must be kept small. In contrast, the optical transformer described herein places a premium on a corner location for the current-feed, amplifying the nonuniformity. Because the untreated sawed edges of the LED chip will cause surface recombination, current cannot be allowed to reach them, so that the LED cannot be illuminated all the way to its edge. It would be an advantage to provide an optical transformer that alleviates luminance inhomogeneities inherent to LEDs.
Beyond making a single source uniform, a better optical method is needed for combining the outputs of spatially separate LED chips, which are easier to cool than when closely packed. Such an optical source-combination device should optimally produce a uniform luminance with sharp edges. Besides easier thermal management, optical source-combination is needed that makes unnoticeable the individual variations or even failures of any of the LEDs.
The LED prior art is also less than satisfactory regarding the geometry of phosphor utilization in LEDs, such as for LEDs that generate white light. A phosphor coating of a quarter-millimeter (250 microns) or more directly onto a one-mm blue chip will necessarily increase source area, sometimes by a factor of four, and thus reduce luminance. The application of phosphor to such small chips necessarily results in color-temperature variations across each chip and between them as well. Also, much of the phosphor output backscatters; that is, it shines wastefully back into the chip, which is relatively absorptive. Finally, the phosphor must withstand the chip's high operating temperature, and differential thermal expansion poses adhesion problems, greatly reducing output if the phosphor should work loose. Although a thinner phosphor layer would have less problem with stress, as well as more luminance, only one manufacturer, Lumileds Corporation, for example, has the advanced phosphor deposition technology for the conformal 25-micron coating of their white LEDs, ten times thinner than the rest. (Laboratory samples from other companies have been exhibited but the processes have not been proven to be commercially viable at this time.) Even these devices vary in color-temperature, across their faces as well as from chip to chip.
It would be an advantage if the phosphor could be situated away from the LED; particularly, it would be an advantage if the phosphor layer in a LED device was positioned remotely enough to be unaffected by the temperature variations of the LED itself. Such a phosphor target could then be as small as the combined area of the separate LED chips, to maximize luminance. Conventional arrays of white LEDs suffer from variations in color temperature. In order to overcome this problem manufacturers employ expensive binning procedures. However, with the current state-of-the-art LEDs, there is still considerable variation in the color temperature, even using tight bins. Further, since an array of close-packed LEDs in practice has a spacing that is typically one or more chip widths between chips, simple application of phosphor over the entire array would result in a diluted, highly uneven luminance.
Achieving higher white luminance from an LED, with uniformity and color-consistency, is critical for LED market penetration into general lighting uses, where the lower power consumption and longer life of LEDs can greatly contribute to energy conservation. Larger and more efficient phosphor coatings can be utilized if they can be separate from their blue-light sources. Such an advance could particularly benefit automotive headlamps, where current white LEDs are marginal at best in luminance. In fact, color temperature variations across a beam could lead to excess blue light, which is ophthalmologically hazardous.
In some applications it is advantageous to produce a number of smaller size sources from a single larger source. This is useful for example when an optical design is difficult to mold because the optical component would be too thick and/or too large. If such a large single source is separated into a number of smaller size sources of the same total area, the same lens design can be used for each such source, just scaled down to a moldable size. It would also be desirable that these smaller sources are more uniform than the larger parent source, or that they have a prescribed luminance output.
In other applications it would be useful to change the shape of a single source or multiple sources to another shape, such as from a square to a rectangle of a substantially equal area or vice versa. This is useful for such applications as LED headlamps where it is desirable to generate rectangular sources with aspect ratios (length to width) of between two to one to six to one. Such a method must, of course, preserve source luminance as much as possible.
Finally, it is desirable to have a highly efficient means of producing white LED light sources without the use of phosphors, by combining two or more LEDs of a different wavelength into a single homogeneous source. Traditionally, the approach has been to use three different colored LEDs to make white light, commonly a red, a green, and a blue LED. However, the traditional optical approaches do not produce a rectangular or square uniform light source using such RGB light sources. It would be beneficial to have means of producing a light source combining more than three LED wavelengths. Additionally, it would be useful to have a means of producing such light sources where the chromaticity of the light source is adjustable.